1. Field of the Invention
This invention relates in general to power supply circuits, and, more particularly, to high efficiency push-pull inverter oscillator or astable multivibrator circuits.
2. Description of the Prior Art
As is well known in the art, the main function of a push-pull inverter is to convert a direct or unidirectional input voltage to an alternating voltage. In most applications of such inverters, it is important that this energy conversion be accomplished with minimal energy loss and power dissipation, primarily to avoid the size and cost disadvantages associated with sizing of components required for inefficient circuitry. Typical push-pull inverter circuits are disclosed in the following U.S. Pat. No. 3,579,026, issued May 18, 1971 to Paget; U.S. Pat. No. 3,663,994, issued May 16, 1972 to Low et al.; U.S. Pat. No. 3,691,450, issued Sept. 12, 1972 to Cox; and U.S. Pat. No. 3,913,036, issued Oct. 14, 1975 to Hook.
Of the various factors which contribute to undesirable energy loss in typical push-pull inverter oscillators, the most significant is the so-called common mode conduction, which occurs when both switching means, usually transistors, conduct simultaneously. This loss factor is the result of the inherent and unavoidable delay associated with the turn-off action of most applicable transistor switching means, which do not have a corresponding delay associated with their turn-on process. Thus, if a simple square wave current or voltage is used for actuating a pair of typical transistor switching means connected in a push-pull circuit arrangement, common mode conduction will invariably result.
The second most significant cause of energy loss is the power dissipation occurring within each transistor switching means during its turn-off transition. In order to minimize this factor, it is important to operate each transistor near its maximum switching speed capability. This maximum switching speed is attained by evacuating the stored charge carriers from the transistor base-emitter junction as rapidly as possible rather than allowing them to dissipate by recombination.
Further energy dissipation in a typical push-pull inverter results from turning on a transistor switching means, that is, rendering it conductive, prior to reduction of its collector voltage to the minimum collector voltage level. This reduction of collector voltage occurs after the other transistor switching means is rendered non-conductive.
Significant energy is also typically lost during the turn-off process of each transistor switching means when the transistor collector voltage rises significantly before the transistor has been turned completely off, that is, rendered non-conductive.
Yet another cause of significant energy loss in a typical push-pull inverter results from power dissipation within each transistor while it is conducting. In order to minimize this loss factor, it is necessary to provide adequate, but not excessive, base drive corresponding to the collector current at any given time.